Orthogonal Frequency Division Multiplexing (OFDM) is a proven access technique for efficient user and data multiplexing in the frequency domain. One example of a system employing OFDM is Long-Term Evolution (LTE). LTE is the next step in cellular Third-Generation (3G) systems, which represents basically an evolution of previous mobile communications standards such as Universal Mobile Telecommunication System (UMTS) and Global System for Mobile Communications (GSM). It is a Third Generation Partnership Project (3GPP) standard that uses scalable bandwidth from 1.4 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations. LTE improves spectral efficiency in networks, allowing carriers to provide more data and voice services over a given bandwidth. Other wireless standards like WiFi (IEEE 802.11) or WiMAX (IEEE 802.16) also employ OFDM.
Among the disadvantages of OFDM, its sensitivity to Doppler, phase noise and frequency mismatches, as well as its large peak-to-average power ratio (PAPR) are among the hardest to combat. Large PAPR signals translate into low efficiency of the power amplifiers (PAs), because the PA operating point should be designed in the linear region hence requiring large back-off values (of the order of 10 dB or more) and consequently low PA efficiencies. This drawback is accentuated at high frequencies because radio frequency (RF) hardware above 6 GHz can present low power efficiency. Sensitivity to frequency misalignments is also critical at high frequencies, as well as the Doppler spread caused by movements of the user and/or the environment which linearly increases with carrier frequency. Phase noise caused by RF oscillators is yet another issue as it brings non-additive impairments at the receiver side which also grow with frequency.
The usual approach to deal with large phase noise and/or large Doppler spreads in multicarrier waveforms is to increase the subcarrier width. However large subcarrier widths lead to channels that can be non-flat inside each subcarrier, hence requiring intra-subcarrier equalization and complicating the receiver's design. On the other hand, subcarrier width is inversely related to the OFDM symbol duration and this has a minimum limit determined by the minimum duration of the basic time transmission interval (TTI), which in turn impacts the numerology of the wireless communications system.
Some solutions to reduce PAPR involve single-carrier waveforms (like single carrier frequency division multiple access, SC-FDMA), which reduce PAPR by several dB, particularly with low-order constellations [4]. Other approaches involve constant-envelope or quasi-constant-envelope waveforms with continuous phase [5]. The approach in [1] comprises an OFDM signal modulating the phase of a constant-amplitude signal. This approach is attractive as it provides better robustness against phase and frequency impairments than OFDM. However, only Additive White Gaussian Noise (AWGN) channels are considered. Mobile radio channels, or wireless channels, usually exhibit a number of highly varying impairments, including the effects of multipath and Doppler spread. Such impairments demand specific techniques at the receiver particularly at very high frequencies.
More adequate waveforms are therefore highly desirable in order to overcome the impact of power inefficiency, Doppler spread, phase noise, and frequency instability in mobile wireless channels.